1. Field of the Invention
The present invention relates to a method of drying an organic pigment, a method of processing a phthalocyanine pigment, and a method of manufacturing an electrophotographic photosensitive member.
2. Description of the Related Art
Hitherto, organic pigments have been used as materials of paints and inks. In particular, organic pigments have been widely used as materials of dispersion liquids (paints) for manufacturing electrophotographic photosensitive members (i.e., organic electrophotographic photosensitive members).
In order to give an electrophotographic photosensitive member with satisfactory electrophotographic characteristics, an organic pigment is particularly important which is used as a charge generating material. The charge generating material imposes a great influence upon the electrophotographic characteristics, such as sensitivity of the electrophotographic photosensitive member, quality of an output image, and a potential variation characteristic when the electrophotographic photosensitive member is repeatedly employed. The chemical structure and the crystal form of an organic material have hitherto been known as factors affecting the electrophotographic characteristics. However, it has recently been confirmed that a dispersed state of the organic pigment in a dispersion liquid used to form a charge generating layer (i.e., a coating liquid for a charge generating layer) is also a factor greatly affecting the electrophotographic characteristics. One of factors affecting dispersibility of the organic pigment in the dispersion liquid is, for example, a method of drying the organic pigment before preparation of the dispersion liquid.
Known methods of drying the organic pigment are practiced by blow drying, heat drying, vacuum drying, or a combination of two or more among them. The hardness and shape of the organic pigment are changed depending on the drying method. As a result, the dispersed state of the organic pigment in the dispersion liquid is also changed.
As techniques for improving dispersibility of the organic pigment, Japanese Patent Publication No. 2-36937 and Japanese Patent Laid-Open No. 10-67946 disclose freeze-drying methods, and Japanese Patent Laid-Open No. 2008-195951 discloses a vacuum heat drying method.
The freeze-drying method is a method of freezing a target substance to be dried and sublimating moisture under depressurization, to thereby dry the target substance. The freeze-drying method is advantageous in that, since no heat is applied to the target substance to be dried, degradation of the target substance is comparatively small. Further, the freeze-drying method is advantageous in that, since the target substance is dried with sublimation from ice, the target substance having fine pores and being superior in dispersibility can be obtained.
The vacuum heat drying method is a method of drying a target substance to be dried by heating the target substance under depressurization.
However, the freeze-drying method has the problem that a drying time is prolonged and a drying cost is increased, thus resulting in lower productivity. Further, even though degradation of the target substance to be dried, e.g., the organic pigment, caused by the freeze-drying method is comparatively small, the degradation is problematic in practice when an organic pigment unsuitable for a freezing process or an organic pigment susceptible to oxidation is the target substance to be dried.
The vacuum heat drying method has the problem that, unless the organic pigment as the target substance to be dried is pulverized before drying, a drying time is prolonged, or the drying is suspended in the middle, thus resulting in lower productivity. The reason is that a hard contracted layer is formed on the surface of the organic pigment during the drying and impedes drying of a deep (core) portion of the organic pigment. In addition, the hard contracted layer formed on the surface of the organic pigment adversely affects dispersibility of the organic pigment in a dispersion liquid.